SKINNY: Fix some warning

Fix: cog [[[END]]] in packet-skinny.h.in
Fix: error: expression which evaluates to zero treated as a null pointer constant of type 'const gchar *' (aka 'const char *') [-Werror,-Wnon-literal-null-conversion] (Reported by Alexis La Goutte and found by Clang 3.6)

Change-Id: Ic354f349eef8a12a7e18e841b517ea4f72ba86f2
Reviewed-on: https://code.wireshark.org/review/3842
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>